Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H2formation

Abstract

Context. We have modelled the abundance distribution and IR emission of the first 3 members of the coronene family in the north-west photodissociation region of the well-studied reflection nebulae NGC 7023.Aims. Our aim was 3-fold: i) analyze the distribution of abundances; (ii) examine the spectral footprints from the hydrogenation state of polycyclic aromatic hydrocarbons (PAHs); and (iii) assess the role of PAHs in the formation of H2 in photodissociation regions.Methods. To model the physical conditions inside the cloud, we used the Meudon PDR Code, and we gave this as input to our kinetic model. We used specific molecular properties for each PAH, based on the latest data available at the present time. We considered the loss of an H atom or an H2 molecule as multiphoton processes, and we worked under the premise that PAHs with extra H atoms can form H2 through an Eley-Rideal abstraction mechanism.Results. In terms of abundances, we can distinguish clear differences with PAH size. The smallest PAH, coronene (C24 H12 ), is found to be easily destroyed down to the complete loss of all of its H atoms. The largest species circumcircumcoronene (C96 H24 ), is found in its normal hydrogenated state. The intermediate size molecule, circumcoronene (C54 H18 ), shows an intermediate behaviour with respect to the other two, where partial dehydrogenation is observed inside the cloud. Regarding spectral variations, we find that the emission spectra in NGC 7023 are dominated by the variation in the ionization of the dominant hydrogenation state of each species at each point inside the cloud. It is difficult to “catch” the effect of dehydrogenation in the emitted PAH spectra since, for any conditions, only PAHs within a narrow size range will be susceptible to dehydrogenation, being quickly stripped off of all H atoms (and may isomerize to cages or fullerenes). The 3 μ m region is the most sensitive one towards the hydrogenation level of PAHs.Conclusions. Based on our results, we conclude that PAHs with extra H atoms are not the carriers of the 3.4 μ m band observed in NGC 7023, since these species are only found in very benign environments. Finally, concerning the role of PAHs in the formation of H2 in photodissociation regions, we find that H2 abstraction from PAHs with extra H atoms is an inefficient process compared to grains. Instead, we propose that photodissociation of PAHs of small-to-intermediate sizes could contribute to H2 formation in PDR surfaces, but they cannot account by themselves for the inferred high H2 formation rates in these regions.